DIET AND LIVER DETERMINE BRAIN METABOLISM AND CONTENT OF ARACHIDONIC AND DOCOSAHEXAENOIC ACIDS In critical reviews, we showed that the nutritionally essential polyunsaturated fatty acids (PUFAs), docosahexaenoic acid (DHA, 22:6n-3) and arachidonic acid (AA, 20:4n-6), are critical for brain and heart function. A coherent kinetic model, applied to experimental measurements in adult rats, showed how brain composition and metabolism of these PUFAs are regulated by dietary intake and/or liver synthesis from their respective PUFAs, alpha-linolenic acid (alpha-LNA, 18:3n-3) and linoleic acid (LA, 18:2n-6). In the absence of dietary DHA, but with sufficient dietary alpha-LNA, the rat liver can maintain a normal brain DHA content by synthesizing 30-times more DHA than the brain consumes. Studies using positron emission tomography (PET) showed that adult human brain consumes AA and DHA at rates of 17.8 and 4.6 mg/day, respectively, and that brain AA consumption does not change significantly with aging (Rapoport and Igarashi, 2009;Rapoport et al., 2010). DIETARY N-6 PUFA DEPRIVATION IN ADULT RATS Arachidonic acid (AA) is critical to brain and body function, but few studies have examined effects of dietary n-6 polyunsaturated fatty acid (PUFA) deficiency. In adult rats fed an n-6 PUFA adequate or deficient diet for 15 weeks, we showed marked alterations in organ lipid composition and metabolism. The deficient diet reduced testes weight, as well as AA concentrations in brain, liver, heart and testes, but increased n-3 PUFA concentrations in these organs. The testes changes reflect the importance of AA in testicular development and fertility (Igarashi et al., 2009). IN VIVO SYNTHESIS-SECRETION OF EICOSAPENTAENOIC AND DOCOSAHEXAENOIC ACIDS Daily human dietary requirements for the long chain n-3 polyunsaturated fatty acids (PUFAs), eicosapentaenoic (EPA) and docosahexaenoic acid (DHA, are uncertain, in part because their rates of liver synthesis from circulating alpha-LNA are unknown. We developed an in vivo method to measure these rates in unanesthetized rats. In a first study, U-13Calpha-LNA was infused intravenously for 2 hours and labeled and unlabeled n-3 PUFAs in arterial plasma were measured using negative chemical ionization gas chromatography-mass spectroscopy. Newly synthesized esterified 13CDHA, 13CEPA, and 13Cdocosapentaenoic acid (DPA) within very low density lipoproteins (VLDLs) appeared in arterial plasma after 60 min, then their concentrations rose in an S-shaped manner as a steady-state secretion was attained. Esterified concentration x plasma volume data were fit with a sigmoidal equation, whose peak first derivatives provided whole body synthesis rates of unlabeled EPA, DPA, and DHA equal to 8.40, 6.27, and 9.84 umol/day, respectively. In a second study, synthesis-secretion of esterified DPA and DHA from unesterified EPA was determined as 2.61 and 5.46 micromol/day. The DHA synthesis rates exceeded the daily rate of brain DHA consumption rate by 30-fold, indicating that liver synthesis from alpha-LNA can maintain brain DHA homeostasis were DHA absent from the diet. This stable isotope infusion method could be used to quantify whole-body PUFA synthesis rates in human subjects (Gao et al., 2009;Gao et al., 2009). QUANTIFYING CONVERSION OF LINOLEIC TO ARACHIDONIC AND OTHER N-6 POLYUNSATURATED FATTY ACIDS IN UNANESTHETIZED RATS Isotope feeding studies report a wide range of conversion fractions of dietary shorter-chain PUFAs to long-chain PUFAs, which limits assessing nutritional requirements and organ effects of arachidonic (AA, 20:4n-6) and docosahexaenoic (DHA, 22:6n-3) acids. Knowing the actual conversion rates can provide critical information for assessing daily nutritional requirements. Using our original in vivo infusion method, we determined rates and coefficients for the conversion of circulating unesterified linoleic acid (LA, 18:2n-6) to esterified AA and other elongated n-6 PUFAs in unanesthetized adult rats on a high DHA but AA-free diet. The steady-state conversion rate of LA to AA equaled 16 micromol/day, exceeding the reported brain AA consumption rate 27-fold. The brain and heart cannot synthesize significant AA from circulating LA, but liver synthesis is sufficient to maintain their homeostatic AA concentrations. The heavy-isotope intravenous infusion method could be used to quantify steady-state liver synthesis-secretion of AA from LA under different conditions in rodents and in humans (Gao et al., 2010).